How did the an app co-founder become an billionaire? The little start-up that could. Why is Facebook paying these two guys $19 billion to Joan the social network when just a few years ago mark Zuckerberg wouldn't even give him a job? Because they founded what's app. It allows users to send unlimited text messages anywhere in the world for free. At least the few eyres. It's actually twice the size of wait per . Facebook is paying $13 million more for what's app than they did inthat cram. If you havened heard of what's a, your kids probably have. And that may be the point. Facebook hack facing a lot of Flook for losing Ou on a younger demographic. They've already gone after other companies including snapshot. 450 million people are using it and a million more are signing up every day. He's connected people of the world. If there's a company that's doing that and might be doing that a little bit better than Facebook, he seems to be going after them. Just last month, the co-founder spoke about his success. We're so overwhelmed by growth. It's kind of mind boggling. The simplicity of our product is really what drives us. He and co-founder poured their life savings into what's app growing it and catching the eye of Facebook found per. Mark Zuckerberg decide head wanted to buy what's app two years ago. The CEO said no thanks. Then on Valentine's day, apparently a change of heart. Koum was ready to sell. Came over for dinner. Interrupted mark Zuckerberg's Valenti Valentine's day and the two of them hammered out the deal. Koum signed the deal at the same where he used to collect food stamps. For now, Facebook will focus on grow growing what's app which will stay independent. Promising no ads, no games and no gimmicks. But once the free-year trial runs out, the money is sure to miles an hour in. With 450 million users and growing, well, you do the math. Thanks for watching ABC news. Tune into "Good morning America"

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